Over the years considerable effort has been expended in the development of processes for the manufacture of printed circuit boards. For example, F. M. Gillett et al, U.S. Pat. No. 2,965,952 issued Dec. 27, 1960 disclose a method for manufacturing etched circuitry. In accordance with this method a plurality of holes are punched through a sheet of dielectric material having relatively thin sheets of conductive material secured to each side. The sheet and annular surfaces of the holes are plated with a conductive material in order to establish electrical connections between the sheets of conductive material. The plated sheets are then coated with an adhesive. A portion of a pattern carried by a decalcomania is used to cover the holes such that the decalcomania overlies the sheet in an aligned manner. Thereafter the decalcomania is pressed into tight contact with the adhesive and the adhesive is allowed to dry. After a backing sheet is removed from the decalcomania and any excess adhesive is removed from the conductive sheets, the composite structure is exposed to an acid etching solution. This etchant removes any conductive material not covered by the adhesively-held decalcomania. When the decalcomania is removed, a printed circuit board having the desired pattern results.
Another illustration of the developing technology in the printed circuit board fabrication art is set forth in a disclosure entitled "Coating of Through-Hole Dielectric Substrates" by H. M. Haddad and H. K. Spannhake appearing in the IBM Technical Disclosure Bulletin, Vol. 9, No. 3, August 1966 at page 227. In this disclosure it is noted that three techniques are available for depositing thick, uniform coatings of photoresist material onto dielectric substrates having through-holes. The first technique is to fill the holes in the dielectric substrate with a water-soluble powder and after the photoresist is applied to dissolve the powder using ultrasonically agitated water. The second technique is to immerse the substrate in water, dry the water from the surface of the substrate, while allowing the holes to remain wetted, and then coating the substrate with hydrophobic photoresist material. The third technique consists of silk-screening the photoresist material through a patternless screen onto the substrate. The photoresist forced through the screen adheres to the substrate in all areas except above the hole areas.
A further illustration of the use of a screening process appears in a disclosure entitled "Screening of Land Patterns" by L. P. Remsen published in the IBM Technical Disclosure Bulletin, Vol. 11, No. 10, March 1969 at page 1341. In this disclosure it is noted that conductive land patterns on ceramic substrates can be formed by screening conductive paste through a screen having the selected pattern. With finer patterns there is a tendency towards voids in the end portion of the lands to which pads of a mounted semiconductor chip are to be joined. To overcome this problem, the mask is removed from the screen in those areas coinciding with the portions of the land patterns to which the pads are to be joined.
Further developments in the printed circuit manufacturing art are exhibited by a disclosure entitled "Producing Nonwettable Lands" by S. A. Milkovich and L. F. Miller appearing in the IBM Technical Disclosure Bulletin, Vol. 12, No. 10, March 1970 at page 1657. The disclosure indicates that a paste comprising powdered metals, dispersed in a binder, can be applied in a desired pattern to the surface of a glazed ceramic by methods such as screening or doctoring. The conductor pattern is fired at an elevated temperature to volatilize the binder to cause the powdered metals to adhere to the substrate. During the firing process, the glaze from the ceramic enters the conductor and acts as a shield to prevent wetting by molten metals, such as those encountered in dip-soldering.
More recent developments in the field are disclosed in B. C. Feng, U.S. Pat. No. 4,022,932 issued May 10, 1977. Feng relates to a resist reflow method for making submicron patterned resist masks. The mask is prepared initially using standard photo or electron beam lithography techniques to yield the smallest aperture dimensions consistent with the state-of-the-art. Then, the resulting mask is placed within a chamber containing an atmosphere of resist solvent vapor. The vapor is absorbed by the patterned resist mask causing controlled resist reflow which uniformly reduces the dimensions of the resist openings by an amount determined by time, temperature, resist thickness, resist type and solvent used.
In a United Kingdom patent application No. GB 2,003,660A published Mar. 14, 1979 there is disclosed a method for depositing an area of a material such as a metal on a substrate. The method includes the steps of defining a resist step on the substrate and then depositing a thickness of the material to provide a uniform coverage of the resist step. Thereafter, the material is unidirectionally etched to define the area as an abutment to the resist step. Finally, the resist is removed to leave the required area of material.
Further developments are illustrated by the disclosure of a flame retardant, flexible, solder resistant cover-coat based on dichloropropyl acrylate in E. D. Feit, U.S. Pat. No. 4,136,225 issued Jan. 23, 1979.
A still more recent development, relating to a process for the production of solder masks for printed circuits, is disclosed in E. Losert et al, U.S. Pat. No. 4,230,793 issued Oct. 28, 1980. This process includes the steps of conveying the boards beneath a free falling curtain of a photopolymer to form a thin layer on a surface of the board. The layer is irradiated with ultraviolet light except for those areas of the surface layer that need to be soldered. The unirradiated areas of the layer are dissolved.
The above discussion serves to illustrate general developments in the art of printed circuit manufacture. At this point the discussion narrows to focus on the specific problem of applying solder masks to printed circuit boards.
A solder mask is a permanent coating applied to a printed circuit board after the conductors have been formed, but prior to the assembly operations of component insertion and attachment. Its purpose is to prevent molten solder from bridging conductors and pads during assembly soldering processes, and subsequently to provide a protective barrier against surface contamination during the circuit's operating life. The solder mask coating is typically applied in a pattern to cover all metallization except for those terminal pads that must remain bare to accept solder for component lead attachment, or to mate with connectors.
The two most common methods for patterning a solder mask coating on a printed circuit board are photoprinting and screen printing. The method used generally is determined by its cost and the resolution and registration accuracy required for the particular circuit. Resolution is a measure of detail delineation. In the case of solder mask the detail of interest is an opening in the coating having a predefined shape. Registration is a measure of the location deviation of the detail from the intended position.
In photoprinting, the entire printed circuit board surface is first completely coated. Virtually all production solder mask photoprinting operations use the solder mask in a dry film format as opposed to a liquid. It is purchased from the supplier already coated and dried (solvent has been removed) on a disposable polyester support sheet. The film of solder mask, as a thermoplastic solid, is applied to the printed circuit board by a lamination process. It is then selectively cured (crosslinked) in the desired pattern using ultraviolet radiation as the energy source. A photomask, containing transparent and opaque areas corresponding to the solder mask pattern, is interposed between the ultraviolet source and the uncured coating to produce selective area exposure. The unexposed, and therefore uncured, portions of the coating are removed by dissolving in a solvent that typically is sprayed onto the surface.
A more complete description of the photoprinting process using dry film solder masks can be found in the following articles: "Photosensitive Dry Film Solder Mask" by Roger N. Brummel and Lyle R. Wallig appearing in The Proceedings of the 19th Electrical/Electronics Insulation Conference, Nov. 11-14, 1975 at pages 15 through 18; "Experiences with Dry Film Solder Mask for P.C.B.'s" by T. J. Hibbs appearing in Proceedings of the First Printed Circuit World Convention, June 5-8, 1978 at pages 2.8.1 through 2.8.8; and "Printed Wiring Design Aspects of Using Permanent Photopolymer (Dry Film Solder Mask) Coatings" by J. J. Hickman appearing in Proceedings of the First Printed Circuit World Convention, June 5-8, 1978 at pages 3.4.1 through 3.4.15.
In screen printing, the solder mask, in an uncured (uncrosslinked) liquid state, is pushed with a moving squeegee blade through a screen mask onto the printed circuit board, which is positioned beneath and in close proximity to the screen. The screen has areas blocked off in a pattern corresponding to the areas on the printed circuit board that are not to be coated. The screen therefore acts as a stencil to allow transfer of the liquid in a predetermined pattern. The application and patterning are accomplished simultaneously.
Additional background information and details concerning the implementation of the screen printing process can be found in the following articles: "Achieving Optimum Performance with UV Curable Solder Masks" by F. Axon, R. Cleek, W. Custer, M. Lipson and D. Mestdagh appearing in Proceedings of the First Printed Circuit World Convention, June 5-8, 1978 at pages 1.4.1 through 1.4.8; "Precision Solder Masks Using Liquid Photopolymer" by R. Suender appearing in Proceedings of the First Printed Circuit World Convention, June 5-8, 1978 at pages 1.5.1 through 1.5.7; and "Using Ultraviolet Radiation Curable Resins for Printed Circuit Coatings " by G. B. Fefferman appearing in The Proceedings of the 19th Electrical/Electronics Insulation Conference, Nov. 11-14, 1975 at pages 11 through 15.
Screen printing is generally the faster and less expensive method, but it is not capable of producing the resolution and registration achievable with photoprinting. With printed circuit board designs moving in the direction of narrower spacing and denser conductor geometries, increasing use of photoprinting is taking place. However, the dry film material cost, and the critical nature of the photomask to printed circuit board registration step are major contributing factors to the high cost of using photoprinted solder mask.
Accordingly, the problem faced by the printed circuit industry is that of applying a solder mask coating to a circuit board by a method that is less costly than photoprinting yet achieves the resolution and registration provided by photoprinting.